Article
Mast cells regulate osteoclastogenesis and bone regeneration under estrogen-deficiency
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Published: | October 26, 2021 |
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Objectives: Among their multiple functions, mast cells (MCs) are best known for promoting allergic reactions; however, MCs are proposed to influence also bone turnover. Increased MC numbers were found in the bone marrow of patients with postmenopausal osteoporosis and inflammatory conditions affecting bone. Supporting, patients with systemic mastocytosis frequently display osteoporosis. Recently, we reported that MCs are involved in ovariectomy-(OVX)-induced bone loss in mice and regulate the inflammatory response and osteoclast activity during bone repair. Here, we studied if MCs are also involved in OVX-induced delayed bone repair using MC-deficient mice. In vitro, we investigated the effects of MC mediators on human osteoclasts dependent on the presence of estrogen.
Methods: Female Mcpt5-Cre R-DTA mice (Cre-: MC-competent; Cre+: MC-deficient; 12-weeks old) were either OVX- or sham-operated and subjected to femur osteotomy. Bone regeneration was analyzed 6h and 21d after fracture by serum, µCT, biomechanical and histomorphometric analyses (n=5-8/group). In vitro, effects of conditioned medium of human mast cell line HMC-1.2 incubated with/without estrogen on human primary osteoclast formation were examined (n=6-16/group), performing also estrogen receptor (ER) alpha RNA interference and MC mediator release experiments (Student's t-test, ANOVA/Dunn's posthoc, p<0.05).
Results and Conclusion: In MC-competent mice, OVX significantly increased the serum levels of IL-6 (OVX vs sham: 998 vs 536 pg/ml) and CXCL10 (OVX vs sham: 48 vs 27 pg/ml) 6h after fracture compared to sham mice. This increased inflammatory response was not observed in MC-deficient mice, as there were no significant differences between sham and OVX mice. On d21 after fracture, bone repair was impaired in MC-competent OVX-mice indicated by a significantly reduced bending stiffness (-46%), bone content (-24%) and size (-36%) of the fracture callus. On a cellular level, osteoblast number and surface were reduced (-32%, -38%, p<0.05) and osteoclast number and activity were increased in the callus of MC-competent OVX-mice compared to sham mice (OVX vs sham: N.Oc/BPm (1/mm) 10.2 vs 7.5, p=0.0108; Oc.S/BS (%) 17.2 vs 11.8, p<0.0001). Notably, none of these differences between sham and OVX mice were found in MC-deficient mice, suggesting that MC-deficiency protects from the negative effects of OVX. In vitro, conditioned MC-medium induced osteoclastogenesis, which was attenuated by estrogen (-48%) mediated via the ERalpha on MCs. Moreover, estrogen reduced the release of the osteoclastogenic factors Midkine (-67%) and CXCL10 (-42%) by MCs.
These results indicate a critical role of MCs in bone repair under estrogen-deficient conditions by regulating inflammation and osteoclast formation and activity. The osteoclastogenic potential of MCs seems to be dependent on the ERalpha on MCs and the release of CXCL10 and Midkine. Hence, targeting MCs might be a future therapeutic option to improve bone regeneration in postmenopausal osteoporosis.